The definite answer is “it depends”.
In general, O has the right to continue running or slow down in a straight line without worrying about 17.8 here as O has claimed all these spots in the space-time before the D had any rights to them. When D chases O directly, D must always be able to avoid a collision when O is merely slowing down—and that usually means that D should not take his eyes off O.
If O changes direction, this changes. Now O can not step in a space which is unavoidable by D as long as D pays reasonable attention to where O is going. Quickly checking for the disc over your shoulder is clearly reasonable if you are running towards an unoccupied position, and contact after that is foul by O by 17.8.
On the other hand, running in a direction without looking where you are going for an extended time when another player is in that general direction is not reasonable and is considered reckless/dangerous play, trumping any foul under 17.8.
In the end, the difference is between “quickly checking” and “looking for the disc for an extended time”.